1. Field of Invention
This invention relates to the manufacture of portland and other hydraulic cements suitable for use wherever a hydraulic cement is utilized. More particularly the present invention relates to a process for the making of portland and other hydraulic cements for such use.
2. Description of the Prior Art
The hydraulic cements represent an important group of cementing materials which are used principally in the construction industry. These cements have the special property of setting and hardening under water. The essential components of the cements are lime (CaO), silica (SiO.sub.2) and components derived from them. In the presence of water, these components react to form, ultimately, a hardened product containing hydrated calcium silicate. The hydraulic cements include portland cement as well as high alumina cement, hydraulic lime, and other lesser known cements.
Of all the hydraulic cements, from a commercial standpoint portland cement is by far the most important, for this cement is a major construction material that is utilized in practically all concrete as well as in most of the masonry mortars. The principal components of portland cement are tricalcium silicate (3CaO.SiO.sub.2), dicalcium silicate (2CaO.SiO.sub.2), and tricalcium aluminate (3CaO.Al.sub.2 O.sub.3), all of which, when in a ground or powdered condition, will react with water to form a hard, stonelike substance held together with intermeshed crystals. A fourth compound, tetracalcium aluminoferrite (4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3), does not exhibit any cementitious properties. The exact composition of portland cement is defined in A.S.T.M. Standard Specifications which are accepted by the industry.
Generally speaking, portland cement is currently obtained by finely intergrinding ground lime and silica and heating the mixture within a rotary kiln to the point of incipient fusion. Incipient fusion occurs at or about 1280.degree. C., depending upon the fluxes that are present in the mixture. The principal fluxes are alumina (Al.sub.2 O.sub.3) and iron oxide (Fe.sub.2 O.sub.3), and these fluxes enable the chemical reactions to occur at relatively low temperatures. Normally the lime is obtained from natural calcareous deposits such as limestone, marl, and aragonite. The silica and fluxes, on the other hand, are normally derived from natural argillaceous deposits such as clay or shale.
Stated briefly, portland cement is made as follows: An argillaceous material and a calcareous material are crushed, mixed and interground to a fine powder. The composition of this mixture must be kept constant within narrow limits, as any wide departure from the optimum will result in an inferior cement. The mixing and intergrinding may be done in the dry condition (the dry process) or it may be done in water (wet process).
The mixture then passes into a rotary kiln where it is heated slowly, due to equipment limitations, to the sintering point. It is essential, however, that regardless of the method of preparation, the mixture before entering the rotary kiln be correctly proportioned, and finely interground. Water and carbon dioxide are driven off before the clinkering zone of the rotary kiln is reached. As the hotter regions are approached, chemical reactions take place between the constituents of the raw mixture. In the course of these reactions new compounds are formed, and some of these melt to partially fuse the charge. The clinker then is caused to drop down into some form of cooler, or is conveyed to a clinker pile where it is cooled, sometimes with a spray of water. When cool, the clinker is mixed with a carefully controlled quantity of gypsum, and the mixture is ground to a very fine powder. That finely ground powder is the portland cement of commerce.
Such a process is not capable of producing the desired product within current technology unless the raw feed materials are first intimately interground prior to introduction into the rotary kiln. Rotary kilns vary in length and diameter. They revolve slowly (one turn in every 1 to 2 minutes) and, as they are slightly inclined, the charge slowly travels downwardly toward the hot end of the kiln. Rotary kilns are heated from the lower end. The hottest temperatures therefore develop in a rather narrow zone at the lower end of the kiln, and become less in the upper end. The maximum temperatures at which rotary kilns are capable of operating are about 1400.degree. C. to 1500.degree. C. At no time does the entire mixture in the rotary kiln become molten. Special refractories are required, especially for the hot zone. Attempting to operate a rotary kiln above the above stated temperature range will result in too high a percentage of the feed mixture becoming liquid at one time and running uncontrollably out of the kiln or in severe damage to the refractories and to the kiln shell.
There is therefore a great need in the industry for a process which will quickly and efficiently cause the necessary chemical reactions and overcome the foregoing problem contrary to the currently used tedious process above-described. It is therefore an object of the present invention to provide an uncomplicated and relatively inexpensive process for rapidly producing portland cement suitable for use in commerce.
Indeed, current cement production is one of the most capital intensive, energy intensive, and environmentally repugnant industries from the standpoint of capital costs, energy costs, and pollution control costs being reflected in the selling price of the final product. The present invention is designed to alleviate these major problems within current cement manufacturing processes.